A lithographic printing plate generally comprises a hydrophobic imaging area, which receives oily ink in a printing process, and a hydrophilic non-imaging area, which receives dampening water. A conventional lithographic process usually comprises steps of masking a presensitized (PS) plate, which comprises a hydrophilic support and a hydrophobic photosensitive resin layer, with a lith film, exposing the plate to light through the lith film, and then developing the plate to remove a non-imaging area with a developing solution.
Nowadays a computer electronically processes, stores and outputs image information as digital data. A presensitized lithographic plate is preferably scanned directly with a highly directive active radiation such as a laser beam without use of a lith film to form an image according to a digital data. The term of Computer to Plate (CTP) means the lithographic process of forming a printing plate according to digital image data without use of a lith film.
The CTP system preferably uses a presensitized lithographic plate that can be attached to a press to print immediately after exposure without a development process.
A method without the development process is referred to as an on-press development method, which comprises steps of attaching an exposed presensitized printing plate to a cylinder of a printer, and rotating the cylinder while supplying dampening water and ink to the plate to remove a non-imaging area from the plate. Immediately after exposing the presensitized plate to light, the plate can be installed in a printer. A lithographic process can be completed while conducting an usual printing treatment.
A presensitized lithographic printing plate suitable for the on-press development method must have a photosensitive layer soluble in dampening water or a solvent of ink. The presensitized plate should easily be treated under room light to be subjected to an on-press development in a printer placed under room light.
Japanese Patent No. 2,938,397 (corresponding to European Patent No. 0770494, and U.S. Pat. Nos. 6,030,750 and 6,096,481) discloses a method for making a lithographic printing plate. The method uses an imaging element (presensitized plate) comprising on a hydrophilic surface of a lithographic based an image forming layer comprising hydrophobic thermoplastic polymer particles capable of coalescing under the influence of heat and dispersed in a hydrophilic binder and a compound capable of converting light to heat. The method comprises steps of imagewise exposing to light the imaging element; and developing a thus obtained imagewise exposed imaging element by mounting it on a print cylinder of a printing press and supplying an aqueous dampening liquid or ink to the image forming layer while rotating the printer cylinder. The imaging element can be treated under room light because the element has sensitivity within an infrared region.
Japanese Patent Provisional Publication No. 9(1997)-127683 and International Patent Publication No. WO99/10189 also disclose a presensitized lithographic plate comprising thermoplastic particles, which can be treated at an on-press development.
Japanese Patent Provisional Publication No. 2001-277740, 2002-29162 and 2002-137562 disclose a presensitized lithographic plate comprising microcapsules in place of thermoplastic particles. The microcapsules contain a thermally reactive compound.